How do I possibly express how badly I feel?

In a heated moment two weeks ago, I hooked up with a good friend despite a cold sore on my lip. I asked him about it, and he said that he didn’t care, and I failed to ask him if he knew what he was dealing with.
Now he has a mouth full of blisters and the most rediculous symptoms of oral herpes that I have ever heard of. I don’t even know what to say, this is clearly my fault.
How can I express to him exactly how sorry I am? Does that even help the situation? What can I do??